Fluid masses frequently form in the bodies of patients both spontaneously and after major operations. Unambiguous and reliable identification of the type of fluid is important for any necessary treatment of the patient. It is important in particular to differentiate between pus, blood, wound fluid (lymph) and urine. Other possible fluids are for example bile, ascites fluid and exudate in the pleural space. The exact composition of such fluid masses can thereby vary within certain fluctuation ranges.
Generally fluid masses in the body of a patient are identified after a computer tomography (CT) examination in the CT recordings. If the fluid masses are deemed to be critical, an invasive diagnostic procedure, e.g. a puncture or operation is currently required, to differentiate the findings further, in particular to determine the type of fluid.
The result of radiographic methods, e.g. computer tomography, mammography, angiography, X-ray examination techniques or comparable methods is primarily to show the attenuation of an X-ray along its path from the X-ray source to the X-ray detector in a projection image. This attenuation is caused by the material through which the radiation passes along the radiation path, so attenuation can also be understood as the line integral over the attenuation coefficients of all volume elements (voxels) along the radiation path. With tomography methods in particular, e.g. X-ray computer tomography, it is possible to use reconstruction methods to calculate back from the projected attenuation data to the attenuation coefficients μ of the individual voxels, thereby achieving a considerably more sensitive examination than when simply considering projection images.
A value standardized on the basis of the attenuation coefficient of water, known as the CT number, rather than the attenuation coefficient, is generally used to represent attenuation distribution. This is calculated from an attenuation coefficient μ currently determined by measurement and the reference attenuation coefficient μH2O according to the following equation:
  C  =      1000    ×                            μ          -                      μ            H20                                    μ          H20                    ⁡              [        HU        ]            with the unit Hounsfield [HU] used for the CT number C. For water a value CH2O=0 HU results and for air a value CL=−1000 HU. As both representations can be transformed into each other or are equivalent, the generally selected term attenuation value or attenuation value coefficient refers below to both the attenuation coefficient μ and the CT value.
Although significantly more information is obtained from an image based on the local attenuation coefficient, problems can arise in individual cases when interpreting an image. A locally high attenuation value may for example be due either to materials with a higher atomic number, e.g. calcium in the bones or iodine in a contrast agent or to a high soft-tissue density, as in a lung node. The local attenuation coefficient μ at location {right arrow over (r)} is a function of the X-ray energy E radiated into the respective tissue or material and the local tissue or material density ρ according to the following equation:μ=μ(E, {right arrow over (r)})=(μ/ρ)(E,Z)×ρ({right arrow over (r)})with the energy-dependent and material-dependent mass attenuation coefficient (μ/ρ)(E,Z) and the (effective) atomic number Z.
The energy-dependent X-ray absorption of a material, which is determined by its effective atomic number Z, therefore masks the X-ray absorption influenced by the material density ρ. Materials or tissue with different chemical and physical compositions, in particular also fluid masses, can therefore have identical attenuation values in the X-ray image. Conversely conclusions cannot be drawn about the material composition of an object under examination from the attenuation value of an X-ray recording.
In the context of this description, unless otherwise specified, the term atomic number is not used in the strict element-related sense but instead refers to the effective atomic number of a tissue or material, calculated from the chemical atomic numbers and atomic weights of the elements involved in the structure of the tissue or material.
It is known from U.S. Pat. No. 4,247,774 in conjunction with computer tomography methods that different X-ray spectra or X-ray quantum energies can be used to generate an image. Such methods are generally referred to as dual spectrum CT. They use the fact that the attenuation coefficient μ is a function of energy based on the atomic number, i.e. they are based on the effect caused by the fact that materials and tissue with higher atomic numbers absorb lower-energy X-ray radiation to a significantly greater degree than materials or tissue with a lower atomic number. With higher-energy X-ray radiation however the attenuation values correspond and are primarily a function of material density. With dual spectrum CT the differences are calculated for example in the images recorded at different X-ray tube voltages.
Even more specific information is obtained, if the so-called basic material analysis method is used with X-ray recordings, as described by W. Kalender et al. in “Materialselektive Bildgebung und Dichtemessung mit der Zwei-Spektren-Methode, I. Grundlagen und Methodik” (Material-selective imaging and density measurement using the dual spectrum method, I. Basic principles and methods), Digit. Bilddiagn. 7, 1987, 66–77, Georg Thieme Verlag. With this method the X-ray attenuation values of an object under examination are measured with lower and higher energy X-rays and the resulting values are compared with the corresponding reference values for two basic materials, e.g. calcium for bone material and water for soft tissue. It is assumed that every measurement value can be shown as a linear superposition of the measurement values for the two basic materials. Thus the proportion of bone and proportion of soft tissue can be calculated from the comparison with the basic material values for every element in the image of the object under examination, so that transformation of the original recordings into representations of the two basic materials results. Basic material analysis and the dual spectrum method are therefore suitable for separating or differentiating predefined anatomical structures or types of material in human and animal tissue with significantly different atomic numbers.
A method is also known from German patent application DE 101 43 131 A1, the sensitivity and significance of which exceed those of basic material analysis and allow for example extremely informative functional CT imaging. With this method the spatial distribution of the density ρ (r) and the effective atomic number Z (r) are calculated by evaluating the spectrally influenced measurement data of an X-ray unit. A combined evaluation of the distribution of density and the effective atomic number allows body components such as iodine etc. to be determined quantitatively and for example calcification to be segmented out based on the atomic number. However the hitherto known methods do not allow reliable determination of the type of fluid in fluid masses in the body of the object under examination.